1. Field of the Invention
This invention relates to the field of air ducts, such as are used in the automotive industry.
2. Description of the Related Art
Air ducts are commonly used, for example, in automobiles where the air duct carries air to an engine. The air may pass through a turbocharger, in which case the air flowing through the ducts may be heated to temperatures of up to 180.degree. C., but in general the air is heated to 140-160.degree. C.
These air ducts are often made by different blow molding techniques. Sequential extrusion blow molding technology is used to produce one-piece air ducts that combine several hard and soft segments.
An example of a conventional air duct portion is in FIGS. 1 and 2, where there is shown an air duct 1 having a bellows 3 that includes a first tube 5 having a plurality of convolutes 7 formed therein, and a second tube 9 connected to one end of bellows 3 and a third tube 11 connected to the other end of bellows 3. Convolutes 7 are, essentially, raised circumferential ridges formed in the surface of first tube 5. The soft material segments are generally positions on straight tube portions and are combined with convolutes to allow air duct deflection and decoupling for assembly, engine movements, shock absorption and NVH (noise, vibration and harshness) control. Decoupling means that the air duct is made such that it moves independent of other parts of an engine that the air duct is connected to. The ends of the air duct, which connect the air duct to the engine, are also made with soft materials to increase the flexibility of how the ends may be connected to the engine.
A problem with conventional blow molded air ducts is that because the bellows is made from soft materials, such as thermoplastic ester ether elastomer (TEEE) or flexible polyamide, the bellows tends to elongate too much in the longitudinal, or axial, direction under temperature and pressure.